Widespread genetic exchange among biotypes has surely occurred historically,
and there is every reason to believe that genetic exchange among wild and
domesticated populations continues with regularity. Figure1.8shows a model of
historical and continuing geneflow relationships between wild and domesticated
groups ofC. sativa.
The above discussion makes it clear that geneflow occurs from domesticated to
wildC. sativa, but what about the reverse? Genetic infiltration from wild popula-
tions into domesticated forms ofCannabishas not actually been documented, but
has been confirmed inHumulus(Small 1980 , 1981 ), and there is no reason why
these two closely related genera should be different in this respect.
Deliberate hybridization among hemp cultivars is documented in the literature.
Hybridization among marijuana strains is poorly documented because of their
illegality, but is nevertheless well known. Sawler et al. ( 2015 ) found genetic evi-
dence of intergradation between the“indica-type” and“sativa-type” forms of
high-THC marijuana, indicative of interbreeding among marijuana biotypes.
As noted above, unless domesticated plants are protected from pollination by
considerable spatial isolation, they cannot be employed for pedigreed seed. Because
domesticated selections are highly susceptible to gene influx from other domesti-
cated selections and from wild-growing forms, to maintain their characteristics they
must be protected from“genetic contamination.”However, gene frequencies in
cultivated plants can also change for the reasons discussed in the next paragraph.
As with many other crops (and domesticated animals), the mutations selected by
humans are usually advantageous to humans but disadvantageous to the plants, and
unless stabilizing selection is practiced, natural selection can result in degeneration
or reversion (sometimes termed atavism) of the genome, with wild characteristics
appearing in cultivated plants. Accordingly, maintaining the genetic purity of a
domesticated cultivar ofCannabisrequires stabilizing selection of desired traits.
Patterns of gene change from various factors are summarized in Fig.1.9. The
extensive intergradation that has resulted from interbreeding is the chief cause of
classification difficulties.
1.9 Sources of Taxonomic Disagreement
with Particular Reference toCannabis
Harlan and de Wet ( 1971 ) remarked“The inconsistencies and lack of agreement
among taxonomists dealing with the same materials are remarkable, to say the
least.”As with all science, disagreement between scientists may be due to relative
competency or knowledge, or simply because an issue is too complex to be clearly
resolved, at least with currently available information. However, in the case of
biological taxonomy, personal psychology of taxonomists often plays a dominating
role in determining the nature of classifications.
38 E. Small